This invention relates generally to frequency measurement systems, and more particularly, to a low-noise system for measuring the frequency in a multiphase electric power transmission system.
A frequency measurement is often required to determine whether the output power of a turbine agrees with the consumer power. With such a measurement, the electromechanical transients and the power balance in the transmission of electric power, illustratively from the turbines of a power generating station to an electric supply network, can be determined. In turbine power generation systems, the frequency is primarily related to the speed of rotation of the driving engine. However, such a speed can be determined directly only in the power generation station itself. Depending upon the type and load status of the consumers, electromagnetic transients occur between the power generating station and the supply network, which transients cause shifts in the phase and frequency of the voltages and currents in the transmission line. Such phase shifts result in incorrect frequency measurements.
The number of phases in an electromagnetic transmission system is only of secondary importance in determining the power balance of the transmission system, and it changes. A low-noise frequency measurement cannot be performed on a single phase and by measurement of a single AC quantity alone, such as through measurement of the time spacing between the zero crossings. Present in the transmission lines, depending upon the type and status of the consumers, are harmonics and subharmonics which shift the zero crossings. It is therefore necessary that the frequency or phase noise occurring in such transmission lines be removed, such as by filtering. In addition, asymmetrical conditions in the network, including asymmetrical short circuits and line interruptions produce difficulties in performing reliable frequency measurements. In such situations, the voltages which are desired to be monitored must be selected, illustratively from a total of six available conductor and conductor-to-ground voltages, in a three-phase voltage system.
It is, therefore, an object of this invention to provide a system which determines and measures, in an electric power transmission system, a quantity available outside of the power generating station corresponding as accurately as possible to the speed of rotation of the driving engine. It is a further object of this invention to provide a frequency measurement system which is not adversely affected by electromagnetic influences, such as subharmonics and harmonics, network faults, and other undesirable influences resulting from the type and status of the consumer.
It is also an object of this invention to provide a system wherein frequency and phase noise can be suppressed by suitable measuring devices, so as to facilitate recognition of critical states of the electromechanical transmission system.